Iran's secretive Quds Force is the elite special operations unit of Iran's Islamic Revolutionary Guard. The most militant wing of the Guard, Quds has reportedly carried out covert operations in Pakistan, Afghanistan, Lebanon and Iraq.
The United States has accused it of aiding insurgent groups behind attacks on U.S. soldiers in Iraq and Afghanistan, Jane's global security site reports.
On Tuesday, the FBI and the DEA announced that they had disrupted a plot to commit terrorism inside the U.S., specifically that elements of the Iranian government were involved in a plan to kill Saudi Ambassador to the U.S. Adel Al-Jubeir.
CNN's Barbara Starr explained the connection between Saudi Arabia and Iran, noting that there has been bad blood between the countries for some time.
Al-Jubeir is close with Saudi King Abdullah and works at the behest of the royal family. He is a visible, highly respected diplomat.
Saudi Arabia has publicly criticized the violence in Syria, Starr explained.
"The Quds Force is essentially looking at Syria as one of its satellite states for the last many years," she said, using the country "as a place from which to launch attacks, to support terrorism ... to run weapons all over the world."
The Quds Force has been a huge concern of the U.S. military, Starr said.
In April 2010, a U.S. Department of Defense report accused the Iranian regime of using the Quds Force to "clandestinely exert military, political, and economic power to advance Iranian national interests abroad," according to Jane's.
The report said that the Quds Force global activities include "gathering tactical intelligence; conducting covert diplomacy; providing training, arms, and financial support to surrogate groups and terrorist (organizations); and facilitating some of Iran's provisions of humanitarian and economic support to Islamic causes."
In August, Maj. Gen. Jeffrey Buchanan said that there had been "a significant increase in support" from Quds to Iranian-backed militia groups that threatened Iraq's security.
Buchanan, the spokesman for U.S. forces in Iraq, said the militia groups now outpace al Qaeda in Iraq, which once controlled much of the insurgency that terrorized the country. He said the U.S. suspects that these groups were behind a number of deaths of American troops in June in which weapons brought in from Iran were used.
The Quds Force's national headquarters are in the southwestern city of Ahvaz, according to Jane's.
The Quds has "offices or 'sections' in many Iranian embassies, which operate as closed sections," Jane's says. "There are indications that most operations are co-ordinated between the IRGC and offices within the Iranian Ministry of Foreign Affairs and Ministry of Intelligence and Security."
The Pentagon issued a report in 2007 that claimed that the Force provided many of the explosives and ammunition for Shiite militia groups in Iraq that attacked coalition and Iraqi government forces, according to Jane's.
Tehran has always denied these claims.